The present invention relates to a method, and also to system, for detecting the presence of predetermined gases or vapours of predetermined concentrations in a monitored area. The invention is particularly useful for detecting hydrocarbon and other gases or vapours when they are present at a concentration which might indicate a possible flammable, explosive or toxic atmosphere.
The increase in the global awareness to the changes in the environment is caused by two major trends observed in the atmosphere, ozone depletion and global warming.
The first alarming change in the atmosphere observed in the late 80's was the hole in the ozone layer and it's continuing depletion caused by various chemicals emitted into the atmosphere, such as chloro-fluoro-carbons (CFC), halocarbons, acids, etc.
The second alarming change in the surrounding atmosphere is the global warming effect caused by the various pollutants released into the atmosphere by industrial processes, creating a "green house" effect whereby infrared radiation from the sun and the Earth is trapped in the atmosphere, causing continuous warming of the environment.
In order to preserve the existing environment, "Clean Air Act" type legislation has been issued World-wide and enhanced the ecological awareness.
In addition to the ecological awareness, the safety of personnel in hazardous environments such as flammable, explosive or toxic atmospheres has also been re-evaluated and criteria for establishing the safety of the environment according to its flammability/explosive or toxic potential have been determined.
Fugitive emissions from various industrial sites have been identified, quantified and permits for allowable concentrations of certain gases have been issued. Lists of flammable, toxic and hazardous gases have been compiled, including their maximum allowable safe concentrations (threshold limit value--TLV, time-weighted average--TWA) and lower limits of explosion or flammability (LEL).
Several areas of environmental awareness can be defined: (i) determination of toxic materials according to "Clean Air Act"; (ii) pollution monitoring (urban and industrial); and (iii) petrochemical unwanted emissions.
In order to facilitate the enforcement of the international legislation and the Clean Air Act, accurate, real time monitoring of various toxic pollutants, CFC, halons (halocarbons), flammable gases, etc., is required.
For facilities operating in today's competitive, highly regulated environment, cost and performance pressures have enforced continuous emission monitoring (CEM) developers towards innovation to improve performance while containing capital and plant operating costs. In-situ to monitoring systems are a cost effective option for applications that require surveillance of a specific component or a chemical family in a gas stream. Novel systems gather measurements either as point concentrations or as an across the area average, using spectral-based analysers.
For process plants that routinely handle toxic or combustible gases, a monitoring system can serve many purposes. For instance adequate monitoring can provide an early warning of a gas leak, which may allow the operators to take the steps needed--either manually or automatically--to reduce the likelihood of fire and explosion, protect personnel, reduce property damage and minimize interruption of production activities. Gas detection systems determine a product release before the resulting vapour is capable of supporting deflagration, detonation or injuring (intoxication) personnel.
To design a monitoring system, the relevant hazards, potential monitoring methods and target location must all be identified. These parameters will vary depending on the facility configuration and type of pollutant under surveillance.
Different monitoring criteria apply to different areas of a process plant. For example in a liquified--gas (LPG) storage area, a gas monitoring system may be needed only to provide an alarm, while in a more populated area of the facility the gas sensing system may be relied upon to initiate process, shut down or actuate air ventilation or water spray systems to dilute the released gas cloud.
In general, more rapid detection of smaller quantities of gas are needed in congested process areas than in open storage areas. For example on offshore platforms the process areas is congested with pipe lines, seals, switches, controllers that pose many potential ignition sources in a rather small area (about 40 m.times.40 m) and in some cases a flash fire not detected in time may cause extreme damage or construction failure. The Piper Alpha offshore oil platform that was destroyed by a catastrophic fire in July 1988 serves as an example.
The requirement of early warning when a gas is detected at, for example, 20% of it's lower explosive limit (LEL) and alarm when the concentration reaches 40% of LEL is a must in today's industrial environment. Most advanced systems, recently introduced into the market offer early-warning gas detection capabilities combined (in the same system) with early flame detection.
For toxic gas monitoring equipment the requirement to detect traces or very small quantities of gas in the air (at concentrations of parts per million--ppm), in order to meet the OSHA/EPA specifications (for TLV, TWA, STEL or LOAEL and NOAEL toxicological indices) initiated the development of very sensitive systems.
Summarizing briefly the gas detection methods employed today in various monitoring and analytical systems, they can be classified in two main groups point detection and remote optical detection methods.
Point detection methods require gas to be drawn from a monitored area, sampled over a period of time, introduced into a dedicated sample cell of the detection system via a probe/pump/permeable filter and analyzed according to one or several of the following methods: resistance temperature detector (RTD); catalytic-combustion; electro-chemical cells; total organic carbon analysis (TOC); flame ionization detector (FID); gas chromatographic; mass-spectrometry; ion mobility spectrometry (IMS); surface acoustic waveguide (SAW); chemical adsorption, such as surface acoustic waveguide (SAW) and surface optical waveguide (SOW); and optical spectroscopy, e.g., in the ultraviolet (UV), infrared (IR) and visible (VIS) spectral ranges.
Remote (open path) optical detection methods rely on "spectral finger-print" absorption pattern of substance/vapour in air to be determined over the optical line of sight open-path in front of the detector or between a radiation source and a detector.
The remote methods are divided in passive and active methods.
In passive methods a detector is calibrated to detect background radiation (from the sun and earth) and identify the spectral absorption of a gas or vapour against this background. FTIR (Fourier Transform Infrared) spectrometry is a well known technique used in monitoring equipment. Several types of FTIR instruments (for laboratory or field applications) are employed for detection of very low concentrations of gases that have an IR absorption spectra. However, these instruments are quite sensitive to tough/extreme environments, are rather big and cumbersome, and require frequent calibration and highly skilled technical operators, extensive spectra library memory, and most important, are expensive for every day industrial monitoring applications.
The requirement for simple, rugged, explosion proof instruments, that can be used in extreme weather conditions and tough envirorunents, that require very simple installation and servicing practices, that are less expensive and cost effective on installation/weight and area coverage, has initiated the development of a new family of remote gas monitoring instruments based on active detection methods.
In active methods an artificial radiation source and a detector are communicating so that a gas passing the line of sight between them would be detected according to its spectral absorbance. Several active methods have been developed in the recent years, most of them analyzing the spectral absorbance of a gas in several spectral bands thus comparing several signals simultaneously. The following lists some of the active methods employed.
Dual optical absorption spectra (DOAS). This method employs the gas analysis in two adjacent bands (reference band where the gas does not absorb and a gas absorption band).
UV/IR. This method employs several spectral bands in the UV and IR bands, thus comparing the substance spectral finger-print in a wider spectral range. The method can also be employed to detect and differentiate between several gaseous substances to be detected.
IR. This method employs spectral analysis of reference/gas absorption bands in the IR band, preferably the near IR, i.e., 1-5 .mu.m.
UV. This method employs spectral analysis of reference/gas absorption bands in the UV band, preferably the solar blind 200-300 nm band.
Since each chemical substance has an unique "spectral finger-print" absorption in the UV, VIS, IR portions of the electromagnetic spectrum and the absorption intensity can be related to the concentration of the chemical substance by the Beer-Lambert Law (see below).
Since the gas absorption and emission spectra are caused by scattering, transmission and absorption of electromagnetic energy due to it's molecular structure, the gaseous spectral finger-print is influenced by the following:
Molecular vibration energy--caused by stretching, bending and rotating of chemical molecular bands. This energy is responsible for the IR spectral finger-print of the molecules.
Molecular transfer of electrons energy--caused by breaking/forming chemical bonds, radicals and charged species changing electrons. This energy emitted or absorbed by a molecule is responsible for its UV spectral finger-print.
Gas detection in production area is aimed at explosion threats. Not all gas clouds are hazardous, only if a flammable cloud plume is wide enough to allow flame acceleration to speeds greater than 100 m/see does it become a significant threat. A flame front needs distance to reach the velocities which cause the damaging effects of over-pressure. This distance is mainly controlled by the confinement and congestion of the area.
In typical off-shore industries a gas cloud of 5 meters diameter can be considered a major threat since it can develop an explosion at a low concentration of 2 LEL (stoichiometric cone.). Traditionally, gas cloud monitoring was achieved by installing a grid of many "point" type detectors, in three dimensional grid formation and correlating their signals. One detector seeing gas should cause a warning while a second one would cause automatic actions. However plumes of significant leaks have passed undetected between monitoring positions, and some sensors saturate to become nonresponsive when really needed.
The requirements of point detectors are stringent: a gas has to reach the detector area (surface) at a concentration high enough to cause an alarm. This is quite difficult to accomplish since the gas dilutes in air because of ventilation which is specifically designed in such high risk areas. Also the number of point detectors that can be installed in an area is limited by the machinery space configuration and maintenance. There is also the inherent restriction of the cost benefit of gas detection due mainly to lack of an so effective action of controlling an explosion with an inadequate number of sensors. Add to these restrictions the operational limitations of present catalytic point detectors, the availability of the new emerging optical beam-(remote) detectors was welcomed.
The beam detectors removed the major problem of the point detectors. No more need exists to install a detector close to the leaking source where the plume of the gas is concentrated enough to detect readily.
With the open-path (remote) beam optical detectors, whether the beam ends are close to the source of leak or far away, provided the whole width of the plume is within the beam, the system's response will be similar.
The present status of off-shore high risk area gas and fire protection includes both types of detection approaches. Point IR detection in congested parts where gas may be trapped for some time and the explosion hazard is significant. Beam (open-path) gas detectors in the major air currents (open spacefence line protection).
The advantages and disadvantages of these technologies are summarized in the following.
Point source detectors are advantageous because they are ideal for use in small confined locations such as air intake to control rooms, generator rooms, pump rooms or other isolated pieces of equipment. They quantify the gas concentration at a given location. They are relatively low cost, commonly used and are well recognized by engineers and maintenance stuff. They are characterized by a simple sensor replacement.
Point source detectors are disadvantageous because they do not reflect the actual gas concentration in the entire area. Some types are subject to poisoning by certain materials, such as silicon compounds, mettalo-organies and halogenated compounds. They are characterized by a slow response. They may not reflect actual hazardous conditions in case of high air flowing conditions, it the flow is not directed toward the detector. The gas must reach the specific detector (accuracy will be comprised if the detector is placed incorrectly, or too few are used). Frequent maintenance is required to check calibration. Operating life may be shortened by the presence of persistent background gases. And, in large open space areas, in order to provide adequate coverage a substantial number of detectors is required.
The introduction of spectral signature analysis in point-source detectors (for example IR point detectors) has reduced some of the disadvantages of point source detectors, however the major drawback of local and limited gas measurement still exists with this type of sensor.
Beam (open-path) gas detectors are advantageous because they provide a direct and fast response to changes in gas concentrations. They provide gas surveillance over a large space. They are characterised by fast responses. The speed of response typically ranges between 0.5 and 10 seconds, which is 5-30 times faster than for point detectors. They are more cost-effective than point-type detectors, if the potential release locations are over a large area such as a row of pumps along a pipe rack. They require low maintenance, since equipment is not subject to poisoning. They provide gas release surveillance over a large area. They are unaffected by high background gas levels. They are substantially unaffected by environmental conditions, such as heat, humidity, snow, rain, etc.
Beam (open-path) gas detectors are disadvantageous because they provide average concentration over a short distance (do not give precise concentration at a given location). The beam emitter must be in line of sight with the receiver or reflector (activity in an area may interfere with the beam, leaving an area without detection until the activity stops). Service of some systems can be costly and time consuming, since replacement of failed sensors requires skilled technicians. External radiation sources may hamper their detection capabilities. And, operation may be impaired due to physical obscuration and other conditions that result in more than 90% reduction in beam signal in cases such as very high fog, however, such failures may be automatically revealed.
Today with the emerging novel techniques of electro-optical monitoring that include smart sensors with specific optical filters and microprocessors algorithms that analyze the absorption signal of a gas component within the cluttered signal of changing environment absorption, the open-path remote sensing (beam-sensors) technology has acquired recognition.
Various petrochemical industries, offshore platforms and oil rigs, storage at chemical facilities, fence-line monitoring of chemical, petrochemical and pharmaceutical plant, paint-booths and paint production and storage areas, compressors and pumping stations, liquefied petroleum gas (LPG) and gasoline filing stations, etc., are better protected by the remote sensing optical gas detectors.
EP 0 584389 A1 (and U.S. Pat. No. 5,281,816) teaches a method and system having advantages in some or all of the above respects and particularly useful for detecting the presence of a predetermined hydrocarbon vapour in a monitored area. According to this method gas in or from the monitored area is exposed to radiation emitted from a flashlamp which emits both ultraviolet radiation and infrared radiation; the ultraviolet radiation is detected within a predetermined ultraviolet spectral range, and the infrared radiation is detected within a predetermined infrared spectral range after the radiation emitted from the flashlamp has passed through the gas; and the detected ultraviolet radiation and infrared radiation are compared with a reference of predetermined attenuation characteristics of the hydrocarbon vapour and concentration in the ultraviolet and infrared spectral ranges.
The system according to EP 0 584389 A1 includes a light source, a beamsplitter and two sensors the signal and reference sensors, each includes a light sensitive element and an optical filter. The optical filter of the reference sensor is selected outside, yet close to, the absorption range of the monitored gas, whereas the optical filter of the reference sensor is selected within the absorption range of the monitored gas. The ratio between the signals obtained from both sensors is used to determine the presence or absence of the monitored gas. The beamsplitter ensures that both sensors sense the same field of view and thereby the noise, or in other words, false positive or false negative indications are reduces.
A product in accordance with the teachings of EP 0 584389 A1 is distributed by Spectronix Inc. under the name SAFEYE. The SAFEYE technology analyzes at least two wavelengths within each spectral band, one in a region where the hazardous gas absorbs and one where it does not absorb. The ratio between theses absorption lines when compared to background spectral absorption lines can provide accurate information with regards to gas concentration (absolute or relative) and the location or migration of a cloud (through various lines of sight).
The absorption intensity is related to the concentration (C) of the gas by the Beer-Lambert law: EQU I(.lambda.)=I.sub.o (.lambda.)e.sup.-a(.lambda.).c.l
where a(.lambda.) is the molecular absorption coefficient at .lambda. and l is the path length. I.sub.o (.lambda.) is the intensity that would be measured in the absence of molecular absorption at .lambda..
The optical path is defined by the location of the transmitter (radiation source) and the receiver (sensor) and possible reflectors therebetween. The spectrally selective analyzer can be at either end. If both the transmitter and receiver are collocated, then either a retroreflector or a topographic target is used to reflect the transmitted radiation back to the receiver.
The SAFEYE gas detector includes two parts: a light source and a receiver at a predetermined distance. The system can detect different gases, with respect to different bandpass filters at the absorbing channel signal and the non-absorbing (reference) channel. The signals are analyzed by the microprocessor included in the receiver.
The radiation source is an unique UV-IR pulsating source that can be activated at various frequencies. The very short pulse of light, nsec, enables the recognition of it's unique pattern by the receiver and distinguishes it from background radiation sources such as sunlight, filament lamps, projectors, heat generators, etc. The receiver contains several sensors according to the specific gases (or chemical families) to be detected.
The signal and reference bandpass filters are centered at .lambda..sub.1 and .lambda..sub.2 in the 3-5 .mu.m IR band or in the 0.2-0.3 .mu.m UV band.
The detector is calibrated via a gas cell that is constructed to contain the gas of interest in between the transmitter and receiver. The system analyzes the spectral-finger-print of a chemical (flammable, explosive or toxic gas) in two spectral bands UV and IR where the monitored gases have defined and unique spectral absorption lines. Specific filters are designed for each spectral channel to identify the gases.
The SAFEYE system can provide fast reliable detection of flammable gases (aromatics or paraffins) at lower explosive limit (LEL) levels as well as identification of low concentrations of toxicants at ppm (parts per million) level. The most advanced version incorporates a fire detection option triple IR, which is highly sensitive to small fires at very long distances (4 times the distance of regular optical fire detectors, i.e., 60 m versus existing 15 m ranges).
This open-path, line-of-sight gas detection system can monitor and transmit an alarm signal prior to occurrence of fire or an explosion, identify the chemical family concerned, and activate the required prevention systems.
Reliability and safety being the most important issues when measuring and monitoring combustible or toxic gases, the following performance criteria's must be addressed by the system. Real time measurement (an active system) over a predetermined transmitter receiver path length. Automatic self-calibration to minimize false alarms. Continuous working through significant interference's such as humidity, rain, fog, snow and background radiation (sun, lamps, heaters etc.). Capability to monitor various gas concentrations from traces to potentially explosive levels (PPM to LEL). Immunity to any chemical reaction with hazardous gas environment. Simultaneous detection of homologue hydrocarbons series (C1-C8) with one instrument. Completely immune to industrial and environmental radiation sources. Easily adapted for field usage, simple installation.
The system of the present invention offers improvements to the system and method described in EP 0 584389 A1 in three directions. According to the first, a unique reference optical filter is employed, which improves the performances of the system and method. According to the second, a unique signal optical filter is employed, which renders the system and method particularly sensitive to a specific gas. Whereas according to the third, three (instead of two) sensors are employed, which obviates the need for a beamsplitter. As a result, as detailed hereinbelow, the system according to the present invention is less affected by water and humidity, dust and debris, and other gases which may mask the detection of a preferred gas.